The invention relates, in general, to a process for applying polyurethane. In particular, the invention relates to a process of applying polyurethane to form an elastomer reinforced substrate. More particularly the invention relates to a process of applying polyurethane to manufacture an elastomer reinforced substrate utilizing VOC-free polyurethane reactants.
The principal method of manufacturing plastic/acrylic signs, bath tubs, sinks, spas, marine parts and the like consists of forming a thermoplastic substrate with a high surface finish or other types of substrates and attaching a secondary layer(s) by spraying a mixture of fiberglass and polyester resin thereon. The sprayed-on material is then subjected to patting and rolling to smooth down the entrapped fiberglass and remove any air pockets. The substrate is then allowed to cure at ambient temperature to achieve a coated fiberglass and polyester resin reinforced substrate. Various materials such as plywood or cardboard may be attached by further sprayed-on applications of polyester resin and fiberglass to achieve desired substrate geometry or additional support for the substrate.
In the past attempts to implement the above discussed manufacturing process have failed due to Environment Protection Agency (EPA) rules and regulations regarding the emission of styrene into the atmosphere. In the case of polyester resin, styrene is used as a cross-linking agent for unsaturated polyesters. Additionally, styrene appears to help foster good bond strength between the polyester resin and various substrates It is estimated by various industry models that 5-10% of the polyester resin weight is released to the atmosphere in typical manual (FIG. 4.) and spray (FIG. 5) applications. Styrene is a volatile organic compound (VOC) and is a suspected carcinogen as well. The industry is currently under EPA mandates to control styrene emissions.
Some of the hazards of styrene monomer are:
flammable liquid and vapor,
may undergo rapid polymerization,
harmful or fatal if swallowedxe2x80x94can enter lungs and cause damage
causes skin irritation,
may cause respiratory tract irritation if inhaled,
inhalation may cause central nervous system effects,
may cause damage to liver,
toxic to aquatic organisms,
causes cancer in laboratory animals.
A contributing factor in the failed attempts to implement the above-discussed manufacturing process is the cycle time. The manufacturing time per unit is typically 25-45 minutes. This manufacturing time must elapse before the reinforced part can be trimmed and fitted for with additional fixtures such as water jets, hoses, handles, etc. Shorter cycle times allow for higher production volumes. Yet, another contributing factor is that the heat generated by the exothermic cross-linking reaction can be quite high and cause deformation of thermoplastic substrates. This often makes it necessary to apply multiple thin layers of reinforcement to thin cross-section substrates.
One attempt to resolve the EPA issues consists of forming a thermoplastic substrate with a high surface finish and attaching a reinforcing layer by a reaction injection molding (RIM) process. The thermoplastic substrate is inserted into a matching, closable mold that provides a space for a layer of reinforcing material. The reinforcing material is comprised of a mixture of a polyol and isocyanate and is then injected into the mold space where the mixture reacts therein. This method is a closed-tool technique that specifies the use of a rigid polyurethane foam or a rigid elastomer with a density of 400 to 800 kg/m3 (25 to 50 lbs/ft3) and is typically injected into the closed mold at a pressure of 345 to 690 kPa (50 to 100 psi). The preferred pressures within the mold are about 80 psi.
There are several disadvantages to the close-tool process. The closed-tool molds and related equipment capable of sustaining the pressures of 345-690 kPa are very expensive. Design changes to this type of mold are also very expensive and limited to retooling or replacing the entire mold. It is very difficult, if not impossible, to achieve a laminate substrate with acceptable dimensions that is of similar strength and hardness as the polyester resin and fiberglass combination utilizing polyurethanes with a specified density of 400 to 800 kg/M3.
It would be desirable to have a process for applying an elastomeric coating to a free standing form or substrate such as plastic/acrylic signs, bath tubs, sinks, spas, marine substrates and the like wherein the process does not emit volatile organic chemicals (VOC) such as styrene. The process would allow the elastomeric coating to be applied and cured quickly, to be applied in multiple layers, without the emission of VOC""s.
The present invention provides a process for applying a polyurethane elastomeric coating to a free standing form or substrate such as plastic/acrylic signs, bath tubs, sinks, spas, marine substrates and the like wherein the process does not emit volatile organic chemicals (VOC) such as styrene. It should be noted that the present invention achieved adhesion of the polyurethane elastomer to the acrylic and co-extruded ABS/acrylic substrates without the use of adhesion-promoting additives to the polyol resin reactant or the isocyanate reactant and no primer on the substrate surface. The process would allow the elastomeric coating to be applied and cured quickly and to be applied in multiple layers. The polyurethane of the present invention is a two-part formulation of a polyol resin reactant and an isocyanate reactant. The reactants are separately proportioned, pressurized and heated, then the proportioned heated reactants are mixed together at the time of application. The proportionate volume ratio of isocyanate reactant to polyol resin reactant is 1:1.05 to about 1:4. The combining of the reactants initializes the formation of the polyurethane with a stoichiometric excess of the isocyanate reactant. The proportioned, heated and pressurized polyurethane reactants are mixed and then sprayed onto a heated rotating substrate. The polyurethane reactants are sprayed onto the substrate in an amount sufficient to create, after curing, a polyurethane elastomer reinforcement layer. The polyurethane reaction continues as the reactants are being sprayed onto the top surface of the rotating heated substrate. The polyurethane reactants polymerize or gel into an elastomer in a time range of 15 seconds to about 120 seconds. If desired the above discussed process may be repeated to add a plurality of layers of polyurethane elastomer to the substrate to achieve varying thicknesses of reinforcement of the substrate.